Plugins add additional functionalities to your WordPress website. The WordPress Twitter Bootstrap CSS plugin adds, as its name tells you already, the Bootstrap CSS to your website. This plugin ships with a built-in Less compiler that enables you to configure Bootstrap to fit your needs by setting up the Bootstrap variables.
You can read more about Bootstrap and how to extend and configure it with Less in Chapter 9, Using Bootstrap with Less. You need to download and install the WordPress Twitter Bootstrap CSS plugin, which can be found at https://wordpress.org/plugins/wordpress-bootstrap-css/. For this recipe, you need to activate a theme that does not use Bootstrap already. You can use the Twenty Fourteen theme shipped with WordPress by default.